The present invention relates to vibration-damping composite sheet steel.
To prevent the occurrence of vibration in the structural and other members of various machines and appliances, vehicles, ships, etc., various methods have been used in the past such as adjusting the installation foundations, increasing the thickness of the members themselves, applying a vibration-damping material to the members and spraying or applying to the members a polymer solution which exhibits a vibration--damping function. Particularly, in view of the recent strengthening in the regulation of noise caused by vehicles, etc., in cities, there has been a tendency, as a countermeasure, toward using materials having a vibration-damping function for the component parts of vehicles, etc., and composite materials including intermediate layers composed of substances having a vibration-damping function have been developed as such materials.
This type of vibration-damping composite sheet steel is made by for example laminating a layer of viscoelastic material, e.g., plastic material of about 0.1 to 0.6 mm thick, for example, between two steel sheets of about 1.0 mm thick, for example, and this construction is intended to effectively perform a function of rapidly converting the imparted vibration energy to heat energy by virtue of plastic deformation of the viscoelastic material layer and thereby effectively damping the vibrations.
In this connection, a comparison between the vibration-damping functions of the vibration-damping composite sheet steel and the ordinary sheet steel in terms of reverberation times shows that in the case of the ordinary sheet steel 500 seconds are required before the vibrations die out to attain a given value as compared with about 1 second required for the vibration-damping composite sheet steel.
It is to be noted that while the vibration-damping composite sheet steel gives rise to no inconvenience but produces great effects so far as it is simply used as a sheet material, various troubles will be caused if the composite sheet steel is for example formed into a desired shape by press working or its surface is subjected to a baking process after the press working.
The inventors have conducted some experiments on the press working of the vibration-damping composite sheet steel into a desired shaped form as will be described hereunder. First, where the vibration-damping composite sheet steel used, as starting steel sheets, steel sheets whose surfaces were not subjected to any surface treatment, there was the danger of causing rust on the bonded surfaces of the steel sheets and the viscoelastic material and separating the bonded surfaces in the course of the actual use.
To overcome this deficiency, the vibration-damping composite sheet steel used comprised starting steel sheets which were subjected to electrogalvanizing and then subjected to a phosphate treatment to improve the adhesion of the coatings. As a result, the occurrence of rust on the bonded surfaces of the steel sheets and the viscoelastic material layer was prevented. However, the occurrence of a blistering phenomenon and hence the separation of the bonded surfaces took place during the baking operation (at 180.degree. C. for 20 minutes) after the press working.
Since this was considered to have been caused by the previously mentioned phosphate treatment of the steel sheets, starting steel sheets subjected only to a surface treatment or electrogalvanization and not to the phosphate treatment were used. However, the rate of occurrence of cracks during press working was increased to exceed 50%, whereas if the viscoelastic material used was for example an ethylene acrylate copolymer resin including carboxyl groups, the same blistering phenomenon as mentioned previously too, place on the bonded surfaces during the baking finish operation after the press working.
From the results of this series of experiments it has been found that the use of the conventional vibration-damping composite sheet steel as a starting blank for press-worked shapes gives rise to some problems as mentioned above.